Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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000
FXUS65 KSLC 190317
AFDSLC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SALT LAKE CITY UT
817 PM MST THU DEC 18 2014

.SYNOPSIS...A WEAK WEATHER DISTURBANCE WILL EXIT THE REGION THIS
EVENING...YIELDING TO A WEAK RIDGE EARLY FRIDAY. AN INCREASINGLY
MOIST NORTHWEST FLOW AIMED AT NORTHERN UTAH IS EXPECTED FOR THE
WEEKEND INTO EARLY NEXT WEEK.

&&

.DISCUSSION...WATER VAPOR LOOP SHOWS A SPLITTING TROUGH PASSING
TO OUR EAST...WITH A RIDGE BUILDING IN FROM THE WEST. AMDAR
400-250MB WIND OBSERVATIONS INDICATE A 65-105KT ANTICYCLONIC JET
FROM OREGON INTO ARIZONA. GOES/GPS/RAP/00Z RAOB INDICATE THE
PRECIPITABLE WATER VALUE RANGES BETWEEN 0.10" NORTHEAST
MOUNTAINS...TO 0.45" DIXIE. BLENDED PRECIPITABLE WATER PRODUCT
SHOWS A PACIFIC MOISTURE TAP INTO THE PACIFIC NORTHWEST...WITH
VALUES OVER 150% OF NORMAL ACROSS MUCH OF THE WEST.

WILL THE TROUGH MOVING TO OUR EAST AND LOSS OF DIURNAL INSTABILITY
THE SHOWERS HAVE ENDED. HAVE REMOVED ALL POPS FROM THE FORECAST. MID
AND UPPER LEVEL CLOUDS CAUGHT IN THE RIDGE AXIS WILL PROPAGATE INTO
NORTHEAST UTAH OVERNIGHT. IN ADDITION...INCREASED CLOUD COVER
ACROSS THE UINTA BASIN AND EASTERN ZONES WHERE 11-3.9U SATELLITE
INDICATES STRATUS WITH INVERTED CONDITIONS IN PLACE.

OTHERWISE LITTLE IF ANY CHANGES TO TEMPERATURES AND WINDS.

PREVIOUS DISCUSSION FOLLOWS...
A WEAK SPLIT SYSTEM CONTINUES TO CROSS UTAH THIS
AFTERNOON...GENERATING ENOUGH INSTABILITY TO ALLOW SPOTTY SHOWERS
TO CONTINUE WITH SNOW LEVELS NEAR 4500FT. EXPECT SHOWERS TO
QUICKLY WANE THIS EVENING DUE TO BOTH THE LOSS OF DAYTIME HEATING
AND THE DEPARTURE OF THE TROUGH. BRIEF CLEARING OF SKIES TONIGHT
BEHIND THE TROUGH WILL ALLOW TEMPERATURES TO FALL ENOUGH TO BRING
A THREAT OF PATCHY FOG TO SOME VALLEY LOCALES.

SHORTWAVE RIDGE TOMORROW WILL BRING A MORE STABLE AIRMASS TO THE
AREA...BUT THIS WILL BE SHORT-LIVED AS A SERIES OF WEAK DISTURBANCES
WILL ARRIVE TOMORROW EVENING. EXPECT ANOTHER ROUND OF SPOTTY SHOWERS
MAINLY ACROSS THE NORTH WITH SNOW LEVELS AGAIN NEAR OR JUST ABOVE
THE VALLEY FLOORS.

WARM AIR ADVECTION AT 700MB WILL STRENGTHEN SATURDAY NIGHT...ABOUT
THE SAME TIME DEEP MOISTURE STARTS TO ARRIVE OVER NORTHERN UTAH.
THIS WILL BE THE BEGINNING OF A PROLONGED WET PERIOD FOR NORTHERN
UTAH. HOWEVER...WARMING AIRMASS WILL LEAD TO HIGHER SNOW LEVELS WITH
MOST VALLEYS SEEING RAIN AND MOUNTAINS RECEIVING RELATIVELY HIGH-
DENSITY SNOW.

IMPRESSIVE ATMOSPHERIC RIVER /AR/ EVENT PUNCHING INTO THE OREGON
COAST WILL TRANSLATE SOUTHEAST AND PEAK OVER THE NORTHERN THIRD OF
THE FORECAST AREA SUNDAY INTO MONDAY. ORIENTATION AND LOCATION OF
THE AR SUCH THAT VAPOR TRANSPORT WILL REMAIN QUITE SOLIDIFIED INTO
THE INTERIOR MTN WEST...NAEFS MEAN INTEGRATED WATER VAPOR TRANSPORT
VALUES CURRENTLY FALLING WITHIN A 15 TO 30 YEAR RETURN INTERVAL
LOCALLY FOR THIS TIME OF YEAR. NOTEWORTHY. THE STRONG UPPER JET AXIS
AIDING INFLUX REMAINS NORTHEAST ACROSS WESTERN WYOMING IN GUIDANCE
HOWEVER...BUT STILL A SOLID 135 KNOTS ALOFT ACROSS NORTHERN UTAH.
WAA PEAKING COINCIDENT WITH PEAK PWAT...0.7 INCHES SUN NIGHT...A KEY
TO THE PROLONGED PERIOD OF ASCENT UPGLIDE AND PRECIP FORECAST ACROSS
THE NORTH THROUGH THAT TIME. VERTICAL PROFILES IN BUFR SOUNDINGS
REMAIN QUITE STABLE THROUGH THIS EVENT...BUT ANOMALOUS MOISTURE
COMBINED WITH AT LEAST SOME DYNAMIC LIFT WITHIN NORTHWESTERLY FLOW
ALOFT IS OWING TO INCREASED CONFIDENCE OF THE BULLISH STORM TOTAL
QPF REFLECTED IN GUIDANCE...IN EXCESS OF 2 INCHES SOME NORTHERN
MOUNTAIN AREAS. CONFIDENCE OF REALIZING OR EXCEEDING THIS REMAINS
HIGHEST FOR THE MTNS NORTH OF I-80 THOUGH SOME AREAS FAVORED BY
NORTHWEST FLOW IN THE CENTRAL WASATCH COULD SEE SIMILAR VALUES. WAA
ADVECTION PEAKS H7 TEMPS TO NEAR -2 C...THUS RAIN/SNOW LINE
THROUGHOUT THE BULK OF THE EVENT SHOULD HOVER AROUND 7500 FEET.
CONTINUED TO BUMP POPS IN THE NORTHERN MTNS AND UPPED QPF IN REGARDS
TO THIS THINKING.

FURTHER AMPLIFICATION OF THE WEST COAST RIDGE MONDAY NIGHT INTO
TUESDAY WILL BREAK THE AR TAP AND ALLOW HEIGHTS ALOFT TO
BUILD...THIS FORCING A RAPID DECREASING TREND OF PRECIP MONDAY
NIGHT. MODELS NOW IN AGREEMENT OF A DRY OR MOSTLY DRY FORECAST
TUESDAY/WEDNESDAY AREAWIDE AND TRENDED POPS IN THAT DIRECTION.

RUN TO RUN AND MODEL TO MODEL AGREEMENT CONTINUES TO RUN AMOK
REGARDING DETAILS OF A STRONGER TROUGH CHRISTMAS EVE NIGHT THROUGH
THE HOLIDAY...THOUGH 12Z RUNS HAVE COME TO BETTER AGREEMENT FOR THE
TIME BEING. GROWING CONFIDENCE IN A SIGNIFICANT COOL DOWN AND SOME
SNOW TO LOW VALLEY LOCALES ALONG THE I-15 CORRIDOR EXISTS...BUT
AMOUNTS REMAIN HIGHLY SUSPECT DUE TO THE POTENTIAL OF A FAIRLY
PROGRESSIVE NATURE OF THE SHORT WAVE UPON PASSAGE.

&&

.AVIATION...SOUTH TO SOUTHEAST WINDS WILL CONTINUE AT THE SLC
TERMINAL THROUGH THE NIGHT. OCCASIONAL CEILINGS NEAR 6000 FEET AGL
ARE POSSIBLE THROUGH 08-10Z.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...NONE.
WY...NONE.
&&

$$

PUBLIC...ROGOWSKI/CHENG/MERRILL
AVIATION...KRUSE

FOR MORE INFORMATION FROM NOAA/S NATIONAL WEATHER SERVICE VISIT...
HTTP://WEATHER.GOV/SALTLAKECITY (ALL LOWER CASE)

FOR FREQUENTLY ASKED QUESTIONS ABOUT THE AREA FORECAST DISCUSSION
VISIT...
HTTP://WEATHER.GOV/SALTLAKECITY/GENERAL/AFD_FAQS.PHP (ALL LOWER CASE)


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